Coating system and its use for producing polyurethane acrylate surface coatings for laminated sheets

ABSTRACT

A coating system comprised of a resin component, a hardening component and additives. The resin component contains (meth)acrylic double bonds and at least two reactive hydroxyl groups, and the hardening component is comprised of a multifunctional, at least bifunctional isocyanate. The proportions of resin and hardening component in the coating system are stoichiometric. The coating system is suitable for producing laminated sheets with a surface layer of progressively hardening polyurethane acrylate. During a first hardening stage, the free hydroxyl groups of the resin component cross-link with the isocyanate groups of the hardening component, forming a urethane acrylate prepolymer which contains (meth)acrylic double bonds. In the second hardening state, the (meth)acrylic double bonds continue to cross-link, forming the urethane acrylic polymer coating of the sheet surface. The progressive cross-linking reaction may be controlled by altering the reaction parameters, such as pressure, temperature, choice of additives and chemical constitution of the hardening component.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a coating system comprised of a resincomponent, a hardener component, and additives, and its use in thepreparation of laminated pressed plates with a surface coatingconsisting essentially of polyurethane acrylates.

2. Description of the Related Art

The use of laminated pressed plates for indoor and outdoor applicationsin the construction sector is known, where the surface of the platesmust meet high requirements with regard to mechanical properties such assurface hardness and scratch resistance. In the case of laminatedpressed plates for outdoor applications, high requirements are alsoimposed regarding resistance to UV radiation and weathering.

According to EP-A-166653, a laminated pressed plate with improvedscratch resistance, namely at least 2 newtons, is provided which has onthe surface of its plates an acrylic resin layer that has been cured byradiation. To manufacture these plates, the outermost web of the carriermaterial, e.g., a web of printed decorative paper, is coated on one sidewith polymerizable acrylic resins so that a solid resin film is created.Then this film is exposed at room temperature to electron beams, causinga radical cross-linking reaction to occur. Finally, this cross-linkedacrylic resin film forming the outermost layer is pressed at highpressure and temperature in a pressing stack of the type customarilyused for laminated pressed plates.

This method is tedious from the standpoint of machine technologyinasmuch as an electron radiation source with a relatively complicatedtype of construction is required in order to achieve the desired surfacehardness of the acrylic resin layer.

In addition, the cross-linked acrylic resin film is difficult tomanipulate in discontinuous pressing processes inasmuch as it tends tocurl easily and not lie flat when the pressing stack is being loaded inlayers. This effect is based on a radiation polymerization induced highacrylate cross-linking density, which is in turn associated withmaterial shrinkage of the acrylate polymers.

The invention is intended to remedy this problem.

SUMMARY OF THE INVENTION

The task underlying the invention is to provide a coating system of thetype described above with which it is possible, in a process that can beeasily carried out, to coat carrier materials and to press them usingthe type of pressing stack customarily used for laminated pressedplates, so that a weather-resistant surface coating with satisfactorysurface hardness and scratch resistance is formed.

According to the invention, a coating system comprised of a resincomponent and a hardener component and additives is proposed which ischaracterized by the fact that the resin component contains not onlyfree, reactive (meth)acrylic double bonds, but also at least tworeactive hydroxyl groups per molecule, and that the hardener componentconsists of a polyfunctional, at least difunctional, isocyanate, wherethe resin and hardener components are present in stoichiometric ratio.

The coating system according to the invention has the additionaladvantage that the resin component has the following general formula:##STR1## where R₁, signifies an alkyl or aliphatic polyether orpolyester group, R₂ and R₃ signify aliphatic or cycloaliphatic groups,and n=2-6, and preferably 3-5; and that the hardener component is anisocyanate with the following general formula:

    R.sub.3 .brket open-st.N═C═O].sub.m

where m is at least 2 and preferably 3, and R₃ signifies an aliphatic orcycloaliphatic residue. Advantageously, the coating system according tothe invention contains as the hardener component a trifunctionalisocyanate whose reactivity at room temperature can also be blocked.

It further contains as additives radical formers that are stable at roomtemperature, as well as inhibitors to prevent a premature cross-linkingreaction of the (meth)acrylic double bonds.

Fillers and/or colored pigments and/or flame retardants and/or agents toprotect against the effects of light such as UV absorbers or radicaltraps can also be present as additives.

Furthermore, according to the invention, a process for manufacturing theabove-mentioned resin components is proposed which is characterized bythe fact that a polyfunctional (meth)acrylate monomer having at leastone free hydroxyl group according to the general formula: ##STR2## wheren=2-6, preferably 3-5, m=0.5-1.5, and R signifies an alkyl or aliphaticpolyether or polyester group, is reacted with a diisocyanate in doublestoichiometric excess in relation to the reactive hydroxyl groups in the(meth)acrylate monomer, with formation of a urethane group, and that thefree isocyanate group then reacts with a polyfunctional, preferablytrifunctional alcohol, with formation of a second urethane group,whereby a resin component containing not only free, reactive(meth)acrylic double bonds but also at least two reactive hydroxylgroups per molecule is formed.

This method can be used preferably to manufacture a urethane acrylateoligomer, and especially an aliphatic urethane acrylate oligomer,containing not only free, reactive (meth)acrylic double bonds but alsoat least two reactive hydroxyl groups per molecule.

The diisocyanate utilized in this process according to the inventionpreferably has functional groups with different reactivity.

It is advantageous to utilize as polyfunctional (meth)acrylate monomersthose having four or five (meth)acrylic double bonds.

In addition, a process for coating carrier materials is provided,utilizing the coating system according to the invention in which thecoating system is applied to one side of a carrier material, and wherethe carrier material which has been coated in such a manner is thendried at an elevated temperature, preferably under 140° C., so that thefree hydroxyl groups in the resin component react with the isocyanategroups of the hardener component, with formation of a urethane acrylateprepolmer coating containing reactive (meth)acrylic double bonds.

By carefully controlling temperature during this process, thepolyaddition reaction occurs between the free hydroxyl groups of theresin component and the functional groups of the hardener component,while the reactivity of the (meth)acrylic double bonds of the resincomponent is preserved. By this means, a flexible coated carriermaterial is produced that is easy to handle and can be stored for asufficiently long period of time.

Suitable carrier materials are slabs of wood, plastic, or metal, andplastic or metal films. Semi-finished products can also be used ascarrier materials, e.g., those based on materials containing fibers,such as paper, which have been impregnated with synthetic resins.

According to the invention, a process is also proposed for manufacturinglaminated pressed plates, consisting of flat carrier materialsimpregnated with synthetic resin plus an outermost layer on at least onesurface of the plate, which consists essentially of a polyurethaneacrylate.

This process is characterized by the fact that a) the coating systemaccording to the invention is applied to one side of the carriermaterial, that the carrier material which has been thus coated is thendried at elevated temperatures, preferably below 140° C., so that aninitial cross-linking reaction of the free hydroxyl groups in the resincomponent with the isocyanate groups of the hardener component occurs,with formation of a urethane acrylate prepolymer containing reactive(meth)acrylic double bonds, that b) a pressing stack made of severalcarrier materials impregnated with synthetic resin is formed, on atleast one side of which the carrier material manufactured according toa) is the outermost layer, and that c) this pressing stack createdaccording to b) is pressed under high pressure and at temperatures abovethe drying temperature, preferably above 140° C., so that an additionalcross-linking reaction of the reactive (meth)acrylic double bondsoccurs, with formation of a urethane acrylate polymer.

By controlling the temperature in the process according to theinvention, the resin component reacts in a two-stage process:

In the first reaction stage, the addition polymerization of the freehydroxyl groups of the resin component with the functional groups of thehardener component occurs, with preservation of the reactivity of the(meth)acrylic double bonds. A premature reaction of these (meth)acrylicdouble bonds is advantageously prevented by adding suitable inhibitors.If an impregnated decorative paper is preferably used as the carriermaterial, the former has a coating with a polyurethane acrylateprepolymer base after the first stage of the reaction which can begrasped and is non-blocking. These coated carrier materials can thusreadily be placed in layers in a pressing stack of the type customarilyused for laminated pressed plates without causing any problems, i.e.,without undesirable curling.

In the second reaction stage, the polymerization reaction of the(meth)acrylic double bonds in the resin component occurs, triggered bythe high temperatures and pressures during pressing. The onset of thepolymerization reaction is also triggered by the addition of suitablefree radical formers.

In the process according to the invention, a trifunctional diisocyanatecan be advantageously utilized as a hardener component, and itsreactivity at room temperature can also be blocked.

Other additives that can be added to the coating system include fillersand/or colored pigments and/or flame retardants and/or agents thatprotect against the effects of light such as UV absorbers or freeradical traps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will be further explained below using some embodiments asexamples:

EXAMPLE 1

a) Preparation of the Resin Component:

100 parts by weight of the polyfunctional acrylate monomer, for exampledipentaerythritol pentaacrylate with an OH equivalent weight of 525, areplaced in a reaction vessel fitted with a stirrer, heater, cooler,reflux condenser, and gas inlet. The diisocyanate used is one that hasfunctional groups with different reactivities, for example isophoronediisocyanate with an NCO equivalent weight of 111. The proportion ofisophorone diisocyanate is 42.3 parts by weight.

0.5% hydroquinone as an inhibitor and 0.1% dibutyl tin dilaurate as acatalyst, plus 20 parts by weight n-butyl acetate as a diluent are alsoadded to the reaction mixture. Dried air is passed into the reactionmixture for the entire duration of the reaction. The reaction mixture isstirred at a temperature of approximately 60° C., until it contains halfas many free isocyanate groups as it did initially (determined usingGerman Industrial standard DIN 53 185). This is caused by the formationof urethane bonds. Then 25.5 parts by weight of trimethylolpropane, apolyfunctional alcohol with an OH equivalent weight of 44.7, are addedto the reaction mixture, which is allowed to continue to react until theisocyanate content has dropped to under 0.5% with formation ofadditional urethane bonds. Then the reaction mixture is cooled to roomtemperature, diluted to approximately 80% with n-butyl acetate, and morehydroquinone in the amount of 0.5% is added. This urethane acrylateoligomer solution has a viscosity at 25° C. of approximately 3000 mPa.s.It can be stored for several weeks with no change in viscosity. Its OHequivalent weight is 564. It can subsequently be used as a resincomponent in the coating system according to the invention.

b) Preparation of the Coating System According to the Invention:

To prepare a coating system according to the invention, 100 parts byweight of a resin component solution according to a) are mixed with 34.6parts by weight of a hardener component in the form of a trimerizedhexamethylene diisocyanate with an isocyanate equivalent weight of 195,and 0.1 parts by weight of tert-butyl perbenzoate are added as a freeradical former. At 140° C., tert-butyl peroxide has a half-life ofapproximately 10 minutes.

The known additives for coating systems such as leveling agents,antifoaming agents, and degassing agents can also be added to thiscoating system. The working life of this coating system is 1 to 1.5hours.

EXAMPLE 2

Preparation of the coating system is carried out as in Example 1. Toprepare the resin component, an alkoxylated pentaerythritol triacrylatehaving a molecular weight of 550 is used as the polyfunctional acrylatemonomer.

EXAMPLE 3

Preparation of the coating system is carried out as in Example 1.Instead of isophorone diisocyanate, however, trimethylhexamethylenediisocyanate is used to prepare the resin component.

EXAMPLE 4

Preparation of the coating system is carried out as in Example 3. Analkoxylated pentaerythritol triacrylate is used instead ofdipentaerythritol pentaacrylate.

EXAMPLE 5

A coating system that can be prepared according to one of the Examples 1through 4 is applied to a carrier material in a coating apparatus bymeans of spreader rolls. The carrier material can be a paper-plasticfilm composite, for example. The coating system is applied at athickness of 40 g/m². Then a partial cross-linking reaction takes placeat temperatures between 100 and 120° C. in the drier of a coatingapparatus between the isocyanate group from the hardener component andthe free hydroxyl groups of the resin component, with formation of aurethane acrylate prepolymer containing (meth)acrylic double bonds.Evaporation of the solvent that was used occurs at the same time. Thespeed at which the carrier material passes through the drier is selectedin such a way that the coated carrier material will be present in theform of a film that can be grasped, which can be stored on supply rollsfor an indefinite time.

To prepare laminated pressed plates with a polyurethane acrylatesurface, it is then possible to cut strips from the supply roll in theproper size for making plates and to layer them with the othercomponents of the layered packet as follows:

coated carrier material, with its coated surface facing toward thesurface of the decorative layer,

decorative layer made of printed or pigmented decorative paper that hasbeen impregnated with melamine resin,

core layer consisting of about 12 webs of paper impregnated with phenolresin,

decorative layer made of printed or pigmented decorative paper that hasbeen impregnated with melamine resin,

optionally one more coated carrier material, with the coated surfacefacing toward the decorative layer.

This press stack is pressed at a pressure of 90 bar and a temperature of140° C. for approximately 20 minutes. At these reaction temperatures,another cross-linking reaction of the reactive (meth)acrylic doublebonds occurs, triggered by the free radical formers which are present inthe coating system, with formation of a cured polyurethane acrylate.Simultaneously there is a curing of the impregnating resins used in thedecorative and core layers. After about 20 minutes, the press stack iscooled down, still under pressure, and then it is removed from thepress. Then the carrier material is removed from the surface of theplate, which now has a cured polyurethane acrylate layer on at least oneside with a scratch resistance of approximately 4N (German IndustrialStandard DIN 53 999, Part 10). The surface of the plate is also highlyresistant to concentrated mineral acid such as sulfuric acid, and tosolvents such as acetone. The weather resistance of the plate was testedaccording to ASTM G 53-84; after 3000 hours, the surface of the platewas found not to have lost any of its gloss or to have becomediscolored.

EXAMPLE 6

A coating system that can be prepared according to one of the Examples 1through 4 is applied in a coating apparatus to a pre-impregnateddecorative paper at a thickness of approximately 100 g/m² by means ofspreading rolls. The pre-impregnation of the decorative paper enhancesthe adhesion of the coating system being used to the decorative paper.Some examples of impregnating resins that can be used for thepre-impregnation are acrylate resins or aminoplast resins, or mixedresins.

During the process, a blade is used to obtain a thickness of 80-100 g/m²for the coating system being applied. The decorative paper that has beencoated in this way then passes through a 20 meter long drying channel at120° C. At these temperatures, an initial cross-linking reaction occursbetween the free hydroxyl groups of the resin component and theisocyanate groups of the hardener component, with formation of aurethane acrylate prepolymer containing still reactive (meth)acrylicdouble bonds.

The reactivity of the hardener component can be further influenced bythe addition of blocked isocyanates that remain inert at roomtemperature but are reactive at temperatures between 100 and 120° C.

In addition, any solvents that may be present in the coating system areevaporated at these elevated temperatures. Thus, a coated decorativepaper results, in the form of a film that can be grasped but isnonetheless flexible, and is resistant to solvents. This flexible filmis rolled into supply rolls and can be kept in storage for an indefinitetime.

To manufacture laminated pressed plates having a polyurethane acrylatesurface, strips can now be cut from the storage rolls in shapesappropriate to the plate and placed in the layered packet with the othercomponents as follows:

coated decorative paper, whose coating consists essentially of apolyurethane acrylate containing reactive acrylate groups

a core layer consisting of about 12 paper webs that have beenimpregnated with phenolic resin

coated decorative paper, whose coating consists essentially of apolyurethane acrylate containing reactive acrylate groups.

This press stack is pressed for about 20 minutes at a pressure of 90 barand a temperature of 140° C. At these reaction temperatures, anadditional cross-linking reaction of the (meth)acrylic double bondstakes place, with formation of the cured polyurethane acrylate. Thiscross-linking reaction is triggered by the free radical formers that arelatently present in the coating system of the decorative layer. It canalso be accelerated by "deblocking" the isocyanates in the hardenercomponent that become reactive only at temperatures of 140° C. andabove.

At the same time, curing of the impregnating resins in the decorativeand core layers occurs. After about 20 minutes, the press stack iscooled down under pressure and removed from the press. The surface ofthe plate now has a cured polyurethane acrylate layer whose scratchresistance is about 4N (DIN 53 999, Part 10). It also is highlyresistant to concentrated mineral acid such as sulfuric acid, and tosolvents such as acetone. The weathering resistance of the plate wastested according to ASTM G 53-84; after 3000 hours, the surface of theplate was found not to have lost any of its gloss or to have becomediscolored.

A completely cross-linked and therefore scratch-resistant surfacecoating based on a urethane acrylate polymer is produced by theinvention. The scratch hardness of this surface coating is at least 2Nmeasured according to EN 438.

The surface coating according to the invention is suitable for bothindoor and outdoor applications. Furthermore, it is resistant tosolvents, so that undesirable graffiti can be easily removed.

We claim:
 1. A process for preparing laminated pressed plates,consisting of flat carrier materials impregnated with synthetic resin,plus an outermost layer on at least one surface of the plate thatconsists essentially of a polyurethane acrylate, comprisinga) applyingto one side of a carrier material a coating system comprised of a resincomponent which contains both free, reactive (meth)acrylic double bondsand at least two reactive hydroxyl groups per molecule, a hardenercomponent consisting of a polyfunctional, at least difunctional,isocyanate, where the resin component and hardener component are presentin stoichiometric quantities and additives and subsequently drying thecarrier material which has been coated in this manner at elevatedtemperatures so that an initial cross-linking reaction of the freehydroxyl groups in the resin component with the isocyanate groups of thehardener component occurs, with formation of a urethane acrylateprepolymer containing reactive (meth)acrylic double bonds, b) forming apress stack consisting of several carrier materials impregnated withsynthetic resin, which has as its outermost layer on at least one sidethe carrier material prepared according to a), and c) pressing the pressstack at high pressure and temperatures above the drying temperature sothat an additional cross-linking reaction of the reactive (meth)acrylicdouble bonds occurs, with formation of a urethane acrylate polymer.
 2. Aprocess according to claim 1, wherein the resin component of the coatingsystem has the general formula: ##STR3## wherein R₁ signifies an alkylor aliphatic polyether group or polyester group, R₂ and R₃ signifyaliphatic or cyclooliphatic groups, and n=2-6; and that the hardenercomponent of the coating system is an isocyanate which has the generalformula:

    R.sub.3 .brket open-st.N═C═O].sub.m

where m is at least 2and R₃ signifies an aliphatic or cycloaliphaticresidue.
 3. A process according to claim 1, wherein a trifunctionaldiisocyanate is used as a hardener component.
 4. A process according toclaim 1, wherein a diisocyanate whose reactivity is blocked at roomtemperature is used as a hardener component.
 5. A process according toclaim 1, wherein an impregnated decorative paper is used as a carriermaterial.
 6. A process according to claim 1, wherein as additives,fillers and/or colored pigments and/or flame retardants and/or agentsthat protect against the effects of light such as UV absorbers or freeradical traps are added to the coating system.
 7. A process according toclaim 1, wherein free radical formers which are stable at roomtemperature and inhibitors to prevent a premature cross-linking reactionof the (meth)acrylic double bonds are added as additives.
 8. A processaccording to claim 1, wherein said coated carrier material is dried atelevated temperatures below 140° C. and said press stack is pressed attemperature above 140° C.
 9. A process according to claim 2, wherein nis 3-5.
 10. A process according to claim 2, wherein a trifunctionaldiisocyanate is used as a hardener component.
 11. A process according toclaim 2, wherein a diisocyanate whose reactivity is blocked at roomtemperature is used as a hardener component.
 12. A process according toclaim 2, wherein an impregnated decorative paper is used as a carriermaterial.
 13. A process according to claim 2, wherein as additives,fillers and/or colored pigments and/or flame retardants and/or agentsthat protect against the effects of light are added to the coatingsystem.
 14. A process according to claim 2, wherein free radical formerswhich are stable at room temperature and inhibitors to prevent apremature cross-linking reaction of the (meth)acrylic double bonds areadded as additives.